1. Field of the Invention
The present invention generally relates to an antenna apparatus used in a radar system mounted on a motor vehicle or the like. More particularly, the invention is concerned with an antenna apparatus or more properly antenna/mirror combination apparatus in which an antenna array is integrally combined with a mirror such as a door mirror mounted on a motor vehicle or the like in such a manner in which at least a part of an antenna element of the antenna array serves also as a part of a reflecting surface of the mirror. Thus, the invention is directed to an inexpensive implementation of the antenna apparatus in a small size while affording a sufficient degree of freedom in design.
2. Description of Related Art
For having better understanding of the invention, background techniques thereof will first be described in some detail. FIG. 21 shows in a sectional side elevational view an antenna apparatus for a car radar system described, for instance, in Japanese Unexamined Patent Application Publication No. 45040/1979 (JP-A-54-45040). In this antenna apparatus, an antenna for the radar system is implemented by making use of a fender mirror of a motor vehicle (i.e., in integral combination with the mirror, to say in another way). Accordingly, the antenna apparatus now under consideration may also be referred to as an antenna/mirror combination apparatus.
Referring to FIG. 21, the antenna/mirror combination apparatus denoted generally by reference character 1A is composed of a mirror case 2 made of a resin material, a mirror 3 having a reflecting mirror surface (not shown) disposed at a predetermined position within the mirror case 2, a feed horn 4 for sending out or radiating transmitting a radio wave to receive echoes, a reflector 5 constituted by a light-transmissive electric conductor for reflecting the radio wave and a transceiver unit 6 for transmitting/receiving radio waves through the medium of the feed horn 4.
The mirror case 2 and the feed horn 4 are mounted on a motor vehicle body 7. The feed horn 4 is disposed in opposition to the reflector 5 and mounted in front of the mirror 3 for radiating the radio wave toward the reflector 5 from a position located outside of the visual field of the mirror 3.
Now, description will turn to operation of the hitherto known or conventional antenna/mirror combination apparatus of the structure described above. The radio wave radiated from the feed horn 4 is reflected at the reflector 5 to be sent out as a radio wave beam of high directivity in the visual field direction of the mirror 3. On the other hand, reflected beam or echo from an object is reflected at the reflector 5 to be received by the transceiver unit 6 by way of the feed horn 4.
On the other hand, the reflector 5 is formed of a material transmissive or transparent to visible light so that the visible light rays impinging onto the reflector 5 are reflected at the mirror 3 to be observed by a driver of the motor vehicle. In this manner, the reflector 5 housed within the mirror case 2 can reflect only the radio waves to play a role only in the transmission/reception of the radio waves without impairing the inherent function of the mirror 3.
FIG. 22 is a perspective view showing in a developed state a hitherto known antenna/mirror combination apparatus disclosed, for example, in Japanese Utility Model Publication 52015/1991. In the case of the antenna/mirror combination apparatus shown in FIG. 22, a thin-structure antenna is housed within a mirror case of a fender mirror or a door mirror of a motor vehicle.
Referring to FIG. 22, the antenna/mirror combination apparatus generally denoted by reference character 1B is comprised of a mirror case 2 and a mirror 3 such as those mentioned previously and a low-profile (thin-plate like) antenna 8 disposed between the mirror case 2 and the mirror 3.
The low-profile antenna 8 is constituted, for example, by a microstrip antenna formed on a substrate and mounted internally of the mirror case 2. In the case of the antenna/mirror combination apparatus shown in FIG. 22, the low-profile antenna 8 is incapable of transmitting/receiving radio waves in the direction in which the mirror 3 is oriented. It is capable of transmitting/receiving the radio waves only in the direction opposite to that covered by the mirror 3.
At this juncture, it should be mentioned that the mirror 3 for the motor vehicle is generally implemented rotatably in order to allow the driver to select the field of view. To this end, an appropriate mirror driving mechanism (not shown) has to be installed within the mirror case 2. Consequently, the mirror case 2 will necessarily have to be implemented in a relatively large structure. Besides, restriction is imposed on the design of the mirror case 2 because of necessity of assuring a space for accommodating the low-profile antenna 8 therein.
As is apparent from the foregoing, in the antenna/mirror combination apparatuses known heretofore, the antenna array is disposed in the vicinity of the mirror 3 or within the mirror case 2 without modifying the structure of the mirror 3 itself. Consequently, the antenna/mirror combination apparatus is necessarily realized in a large size because of bulky peripheral structure, presenting thus a problem that not only the miniaturization of the antenna/mirror combination apparatus but also low-cost manufacturing thereof is difficult or practically impossible.
In particular, when the feed horn 4 is disposed in the vicinity of the mirror 3, as in the case of the antenna/mirror combination apparatus shown in FIG. 21, there arises the necessity of installing the reflector 5 in addition to the mirror 3. Thus, the problem mentioned above becomes more serious.
On the other hand, in the case of the antenna/mirror combination apparatus shown in FIG. 22 in which the low-profile antenna 8 is disposed within the mirror case 2, the space required for installation of the mirror driving unit within the mirror case 2 is subjected to limitation due to disposition of the low-profile antenna 8, giving rise to another problem that the degree of freedom in design of the mirror 3 is remarkably degraded.
Furthermore, because the mirror 3 provides an obstacle to the low-profile antenna 8, transmission/reception of the radio waves is rendered impossible in the direction in which the mirror 3 is oriented, incurring a further problem.